The subject of the Invention is a sound generator that generates directional low-frequency useful sound via a modulated ultrasonic beam. On the other hand, conventional sound generators (such as loudspeakers, sirens, air-modulated devices, etc.) essentially function as monopole sources. As a rule, loudspeakers require a large-volume housing for acoustically effective radiation with low frequencies. Directional radiation at medium and low frequencies is only possible using a cumbersome array set-up of several monopole sources with expensive, frequency-dependent control of the individual monopole sources being required, however. The object of the invention at hand is creating a sound generator having small dimensions that operates along an adjustable virtual array having any length and thereby making extremely directed useable sound radiation possible. In accordance with the invention, the ultrasonic generator emits an ultrasonic cone having carrier frequency .OMEGA. which is also modulated with modulation frequency .omega., with .OMEGA. being greater than .omega.. The beam angle of the ultrasonic cone is assumed to be small in the following, so that the transverse dimensions of the cone within the effective range of the ultrasonic sound are small a compared with the wavelengths to be radiated. During propagation, ultrasonic power N.sub.o emitted by the ultrasonic generator diminishes exponentially as a result of absorption. The sound power modulated harmonically with frequency .omega. along the ultrasonic beam is as follows, taking the transit-induced retardation into consideration: ##EQU1## with: N(x,t): Sound power along the ultrasonic cone
N.sub.o (t): Sound power emitted by directional transmitter PA1 x: Path coordinate in propagation direction PA1 t: Time PA1 c: Velocity of sound PA1 x/c: Transit time-induced retardation PA1 .alpha.Absorption coefficient with carrier frequency .OMEGA. PA1 p.sub.o : Ambient pressure PA1 r: Distance from the directional transmitter to the test point PA1 .theta.: Angle between test point and ultrasonic beam
Ultrasonic power can be modulated in various ways. Thus, the ultrasonic amplitude of the carrier signal can be modulated. Depending upon the degree of modulation, undesired ambient noise can occur, which can be prevented using known measures (such as predistortion, etc.). Another possibility is frequency modulation, for example via two ultrasonic generators oscillating at different frequencies. The ultrasonic power can also be modulated by modulating carrier frequency .OMEGA. and, thus, the absorption coefficient .alpha.. In doing this, it must be taken into consideration that the absorption coefficient does not depend linearly on the carrier frequency. The modulation can also be carried out by influencing the ultrasonic sound reactively or resistively, for example by using resonators and/or absorbers. The variation types of modulation can be combined. The absorbed ultrasonic power along distance dx is as follows: ##EQU2##
The absorbed ultrasonic power dN.sub.Abs (x,t) produces local warming and a volume change of the ambient medium (monopole radiation) as well as radiation pressure which exerts a force on the ambient medium (dipole radiation). The source strength of the monopole dQ(x, t) and the force dF(x,t) of the dipole are as follows: ##EQU3## with: K: Adiabatic exponent of the ambient medium
The useful sound pressure components of the monopole and dipole sources superpose producing an amplification in the direction of the ultrasonic propagation. In the opposite direction weakening of the useful sound radiation occurs. In the case of an ultrasonic cone, referred to as "ultrasonic beam" in the following, this acts like a long virtual array of individual monopole and dipole sources due to the absorption which is only gradual. Characteristic array length L and half-life distance L.sub.0.5, (within which up to one half of the ultrasonic power is absorbed are determined by the absorption coefficient .alpha.. ##EQU4##
The absorption coefficient is .alpha.=0.03 to 1 m.sup.-1 for ultrasonic frequencies .OMEGA.=10 to 200 kHz, which corresponds to a characteristic array length adjustable from L=33 to 1 m. Owing to the transit time of the ultrasonic beam, the areas of the array radiate to each other in a time-displaced manner, producing strongly directional useful sound radiation in the propagation direction of the ultrasonic beam ("end fired line" Olson, Elements of Acoustical Engineering, Nostrand Company, Mc. Princeton, 1957). Overtones can be used in a concerted manner in order to increase absorption and thereby reduce characteristic array length L. The possibility of using broad band ultrasonic sound as a carrier also exists in addition to a single or several carrier frequencies. The resulting useful sound pressure at a test point in a free field (far field approximation) follows for an effective array length l: ##EQU5## with: .sigma.: Equals density of air
Useful sound pressure p is retarded, on the one hand, by time x/c (transit time of the ultrasonic sound from emission point x=0 to radiation location x) as well as by time (r-x cos .theta.)c (transit time from radiation location to test point). The following formulas are given in general for the asymptotic case 1.fwdarw..infin.. The following is produced for the useful sound pressure (far field approximation) with absorbed sound power dN.sub.abs (x,t): ##EQU6## The directivity characteristic R follows: ##EQU7##
A useful sound frequency-dependent carrier frequency .OMEGA. makes it possible for the ratio of the characteristic array length L to the useful sound wave length .lambda. and thus the useful sound directivity characteristic R to be the same with all frequencies. In contrast to the case of a free field, with tube installation, the useful sound pressure amplitude in the emission direction of the ultrasonic cone is independent on angular frequency .omega.. In calculating the free-field characteristic it was presumed that the ultrasonic sound propagates along a beam. This model is sufficient as long as the cone width of the beam is small as compared with the wave length of the released useful sound. In the case of larger cone widths, an additional directional effect occurs due to the sectional perpendicular planes that are vibrating almost in-phase to the propagation direction. This directional effect is all the greater, the greater the local ratio of the ultrasonic cone width to the modulation wave length becomes. This directional effect is amplified if several parallel offset ultrasonic generators are used. The forward/reverse ratio of the useful sound is as follows: ##EQU8##
An additional monopole source can be used for influencing the directivity coefficient. The additional monopole can also be realized directly at the emission location by partial absorption of the ultrasonic sound. Another possibility consists of influencing the reverse dipole radiation using structural measures, such as encapsulation. Owing to the short ultrasonic wave lengths, this can be accomplished using small-volume measures. If the directional transmitter is installed in a tube, the resulting useful sound pressure (one-dimensional wave propagation being presumed) is calculated as follows: ##EQU9##
Due to the fact that the directional transmitter does not function as a point source, rather it radiates along a virtual array, depending upon the absorption coefficient or carrier frequency, bundling of the wave propagation (one, two, three-dimensional sound field) etc., the useful sound pressure level in a free field does not drop proportionally 1/r in the proximity of the ultrasonic source as is the case with conventional sound generators. On the other hand, the useful sound pressure amplitude can possess any desired course in the propagation direction. It can drop, be held constant over a certain distance, or increase or possess a maximum in a certain distance. In the case of one-dimensional wave propagation (a tube for example), the useful sound pressure amplitude increases with the distance to the emission point. Piezoelectric sound generators are used in order to generate high ultrasonic power, these sound generators are coupled to resonators to increase the radiated power (air ultrasonic vibrator). In addition to the ultrasonic generators that are known per se, pneumatic ultrasonic generators such as the Galton whistle, Hartmann generator, Boucher whistle, vortex whistles, Pohlmann whistles and ultrasonic sirens for generating ultrasonic power are particularly suited. The subject of the invention is explained in more detail on the basis of the embodiments.